1. Field of the Invention
This invention relates to a method for controlling flow rate of refrigerant in the refrigerating cycle, more particularly to a method for controlling refrigerating systems having an expansion valve as an electric refrigerant flow rate controlling means to keep constant the degree of superheat at the outlet of an evaporator.
2. Description of the Related Art
A pressure reducing device called an electric expansion valve or electronic expansion valve is used as an electric refrigerant flow rate controlling means in the refrigerating cycle. FIG. 1 shows the construction of the prior art refrigerant flow rate control apparatus.
The refrigerant flow rate control apparatus of FIG. 1 includes compressor 1, condenser 2, electric expansion valve 3, and evaporator 4. In the case where an electric refrigerant flow rate controlling means is used, compressor 1 may be so designed that the rotation speed thereof can be changed by means of an inverter. The degree of opening of electric expansion valve 3 can be set in response to an electric signal. The apparatus further includes first temperature sensor 5 for sensing the evaporating temperature, second temperature sensor 6 for sensing the superheated vapor temperature at the outlet of evaporator 4 and control circuit 7.
In the refrigerant flow rate controlling method using the apparatus of FIG. 1, the control operation is generally effected to keep constant the temperature difference between the evaporating temperature in the evaporator and the superheated vapor temperature at the evaporator 4 outlet in various cases including the case where a plurality of objects are to be controlled, for example, where the rotation speed of compressor 1 or the loads of evaporator 4 and compressor 2 are controlled. That is, a difference between measurements by first and second temperature sensors 5 and 6 is first detected, then a temperature difference .DELTA.T between the detected temperature difference and a preset superheated refrigerant temperature is detected, and control circuit 7 is operated to supply an output signal corresponding to detected temperature difference .DELTA.T to electric expansion valve 3, thus setting the degree of opening or the valve opening of electric expansion valve 3 to control the flow rate of refrigerant.
The control method has been adopted to make the best use of the function of a thermostatic expansion valve.
The electric expansion valve 3 is utilized since it is considered that the method for controlling the electric expansion valve 3 can be made more advantageous than that for controlling the thermostatic expansion valve which was formerly used.
For example, a control signal for the thermostatic expansion valve is a pressure difference. In contrast, a control signal for driving the electric expansion valve 3 is an electric signal. Therefore, it is possible to generate a first signal (which is hereinafter referred to as a proportional term) obtained by multiplying detected temperature difference .DELTA.T by first proportional constant K1, second signal (which is hereinafter referred to as an integration term) obtained by integrating detected temperature difference .DELTA.T with respect to time and multiplying the integrated value by second proportional constant K2, and third signal (which is hereinafter referred to as differentiation term) obtained by differentiating detected temperature difference .DELTA.T with respect to time and multiplying the differentiated value by third proportional constant K3, thus permitting PID control to be effected. In contrast, the control operation for the thermostatic expansion valve is based on proportional control operation.
PID control operation can be represented by the following equation: ##EQU1## where E: a valve opening specifying signal supplied to the electric expansion valve,
.DELTA.T: detected temperature difference (T2-T1)-.DELTA.T0, PA0 T1: temperature detected by the first temperature sensor, PA0 T2: temperature detected by the second temperature sensor, PA0 .DELTA.T0: preset temperature of superheated refrigerant,
K1: proportional constant of the proportional term, PA1 K2: proportional constant of the integration term, PA1 K3: proportional constant of the differentiation term.
As described above, in the case where the electric expansion valve 3 is used to control the degree of superheat of refrigerant by the refrigerant flow rate controlling method, the integration term of K2.multidot..intg.(.DELTA.T)dt and the differentiation term of ##EQU2## are used as control factors in addition to the proportional term of K1.multidot..DELTA.T. Therefore, it has been considered that more reliable control can be attained in this case in comparison with the case where the thermostatic expansion valve is used.
In the actual control operation, however, it is sometimes impossible to control the degree of super-heat in the refrigerating cycle and the response characteristic with respect to the fluctuation in the load may happen to become extremely deteriorated.
In order to solve the problems, the following proposals have been made in the prior art.
(1) It has been considered that the degree of refrigerant superheat in the evaporator cannot be correctly controlled when the load condition, for example, the temperature in the refrigerating chamber is greatly changed. The reason therefor is as follows:
The rate of refrigerant flowing through the electric expansion valve 3 set at the preset degree of valve opening in the refrigerating cycle is a function of pressure difference .DELTA.P between pressures on the high and low pressure sides in the refrigerating cycle. At the same time, the flow rate of refrigerant is a function of the degree of valve opening. Therefore, a group of curves showing the relation between the refrigerant flow rate and the pressure difference between the high and low pressures can be obtained when the degree of valve opening is changed as a parameter.
Assume that the degree of valve opening increases from E.alpha. to E.beta. according to equation (1) due to variation in detected temperature difference .DELTA.T when the refrigerating cycle is effected with pressure difference .DELTA.P1. Assume that, at this time, increase in the refrigerant flow rate is .DELTA.M1. Further, assume that the operation point in the refrigerating cycle is varied to change the pressure difference at the operating point to .DELTA.P2. At this time, the degree of valve opening is changed from E.alpha. to E.beta. based on equation (1) due to variation in detected temperature difference .DELTA.T in the same manner as described above. In this case, variation in the flow rate is .DELTA.M2. However, since the curve of relation between .DELTA.P and .DELTA.M is changed with the degree of valve opening as a parameter, refrigerant flow rates .DELTA.M will be changed even if detected temperature differences .DELTA.T are the same.
Thus, it is considered that the undesirable response characteristic is caused by the fact that, since the flow rate characteristic of the flow rate control valve in the operating conditions as described above is not taken into consideration, the amount of variation in the refrigerant flow rate may greatly differs even if the same instructions for controlling the degree of valve opening of the electric expansion valve 3 are given.
According to this consideration, the amount of variation in the refrigerant flow rate may be changed depending on the operating condition if the coefficients in the proportional, integration and differentiation terms of a signal of determining the degree of valve opening are kept constant irrespective of the operating condition in the refrigerating cycle. Therefore, a control method is proposed in which the condenser temperature is detected, and the operating condition in the refrigerating cycle is determined by the detected condenser temperature together with the temperature at the evaporator inlet to correct the coefficients (refer to Japanese Patent Disclosure No. 61-89454).
(2) Further, the following proposal has been made with much attention taken to the similarity of the electric expansion valve to the thermostatic expansion valve 3. For example, in a system of FIG. 2 in which the rotation speed of compressor 11 is fixed, if a rise in the ambient temperature or an increase in the thermal load of evaporator 14 occurs to cause a disturbance to be input into the control system, the vapor temperature slightly rises and the temperature of the refrigerant at the outlet is greatly enhanced. In this case, the heat capacity of the evaporator 14 is large and therefore the response time is delayed. This causes the degree of refrigerant superheat at the outlet to be increased with time delay, and the degree of valve opening of electronic expansion valve 13 is set larger by means of control circuit 16 and pulse motor 17 according to a signal from subtracter circuit 15, increasing the flow rate of refrigerant from condenser 12. However, it takes a long time for the effect of an increase in the flow rate of the refrigerant to be transmitted to the outlet of evaporator 14 because of the transfer lag of the refrigerant in evaporator 14 and the heat capacity. Further, time delay occurs in the operation of temperature sensor 18 for temperature detection of the refrigerant in evaporator 14.
That is, an unstable condition is set up in the feedback control system for controlling the refrigerant flow rate due to the response lag in evaporator 14 and temperature sensor 18.
It is considered that the unstable condition can be eliminated by inserting a time delay compensating element into the valve driving electric circuit in the case where the electronic expansion valve 13 is used.
This consideration derives out an idea of utilizing the technical concept of phase lag-lead in addition to the technical concept of the PID operation used in the consideration (1). The technical consideration has produced the following proposal.
In response to inputs corresponding to variations in the thermal load and the refrigerant flow rate, the refrigerant flow rate is feedback-controlled by a refrigerant superheat signal representing the degree of refrigerant superheat at the evaporator outlet and having a large time delay. Therefore, the superheat response characteristic is deteriorated. This problem may be solved by combining a feedforward control system with the feedback control system for controlling the flow rate in the electronic expansion valve by a superheat signal. The feedforward control system serves to directly control the flow rate in the electronic expansion valve 13 by a temperature signal of air supplied from the exterior into the evaporator 14 for heat exchange. In this case, the temperature signal of air may be generated from a thermistor. With this countermeasure, the response speed can be made high to improve the super-heat response characteristic to a great extent.
However, in the two types of technical considerations described above, the superheat signal of the electric flow rate controlling and pressure reducing means is dealt with in the same manner as the superheat signal in the thermostatic expansion valve. That is, the substantial nature of the superheat signal is not taken into consideration and the superheat signal is used as it is and combined with other additional signals to improve the control characteristics. This makes the control system rather complicated and cannot attain the desired purpose.
In practice, even with the complicated control system, the control system having the electric expansion valve 3 has not been realized which is far superior to that having the thermostatic expansion valve from the view point of superheat control.
It has been also proposed to replace the classical PID system by an excellent control system including a sample-hold type optimal regulator using a computer. However, in this case, it is not considered to improve the nature of the superheat signal.